Several types of display devices are known which are switchable between a public display mode and a private display mode, with varying degrees of additional cost over a standard display, ease of use and strength of privacy performance.
Devices incorporating such displays include, for example, mobile phones, tablet and laptop computers, desktop monitors, Automatic Teller Machines (ATMs) and Electronic Point of Sale (EPOS) equipment. Such devices can also be beneficial in situations where it is distracting and therefore unsafe for certain viewers (for example drivers or those operating heavy machinery) to be able to see certain images at certain times, for example an in-car television screen while the car is in motion.
Image processing methods exist for producing a privacy effect in liquid crystal displays (LCDs) which, when in the private mode manipulate the image data in a manner dependent on a second, masking, image, and therefore cause that masking image to be perceived by the off-axis viewer when the modified image is displayed. Examples of such image processing methods are given in Powell et al., GB2428152A1, published on Jan. 17, 2007; Broughton et al., WO2009110128A1, published on Sep. 11, 2009; Broughton et al., WO2011034209, published on Mar. 24, 2011; and Broughton et al., WO2011034208, published on Mar. 24, 2011. These methods provide an electronically switchable public/private display with no additional optical elements required, minimal additional cost, and satisfactory privacy performance. These methods all utilise the limited resolution of the human visual system by redistributing the luminance produced to the on-axis viewer by a group of neighbouring pixels within that group while maintaining the same overall luminance produced by the group as a whole. These methods may be described as “pixel splitting”. However, these image processing methods also rely on the off-axis to on-axis luminance curve being strongly non-linear, as illustrated in FIG. 2, so that pairs of pixels in which splitting has been applied produce the same on-axis luminance, but significantly different off-axis luminance to a pair of pixels in which no splitting has been applied This is not the case for some display modes, for example, in-plane switching (IPS), fringe-field switching (FFS) LC display modes and OLEDs. This is illustrated in FIG. 3 for an FFS mode LCD, which shows the difference in off-axis luminance between pairs of pixels with and without splitting applied is at best 10%. As a result, the privacy effect is not sufficiently strong in these display devices to disguise the on-axis image from an off-axis viewer.
Smith et al, U.S. Pat. No. 8,368,727 B2, published on Feb. 5, 2013, describes methods in which a switchable privacy effect can be enabled in displays of this type, having close to linear off-axis to on-axis luminance response, by engineering individual pixels within each group of pixels to have differing view angle properties, and thereby providing a stronger privacy method then achievable with the methods of WO2009110128A1. The methods described in Smith et al. suggest the use of lenticular arrays or parallax barriers to modify the inherently uniform, close to linear off-axis to on-axis luminance response of the pixels in these types of display, so as to result in two pixel types with differing view angle performance. An example pair of modified off-axis to on-axis luminance responses for the two pixel types, and the basis of the corresponding switchable privacy method of U.S. Pat. No. 8,368,727 B2 are illustrated in FIG. 4. It can be seen that in contrast to the methods of WO2009110128A1, a privacy effect is generated by contrasting regions with pixel splitting in one direction (i.e. concentrating the combined luminance of a pair of pixels into a first pixel of the pair having a first type of modified off-axis to on-axis luminance response), against regions with pixel splitting in the opposite direction (concentrating the combined luminance of a pair of pixels into the second pixel of the pair, which has a second type of modified off-axis to on-axis luminance response.